HR: 17:15h
AN: U42B-06    [PDF]
TI: Do Lunar Soils Tell us When the Geomagnetic Field First Appeared?
AU: * Ozima, M
EM: EZZ03651@nifty.ne.jp
AF: Department of Earth Planetary Sciences, University of Tokyo, Bunkyo-Ku, Tokyo, 113-0033 Japan
AU: Miura, Y N
EM: yayoi@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, Bunkyou-Ku, Tokyo, 113-0032 Japan
AU: Podosek, F A
EM: fap@levee.wustl.edu
AF: Department of Earth Planetary Sciences, Washington University, One Brookings Drive, St. louis, MO 63130 United States
AU: Seki, K
EM: seki@stelab.nagoya-u.ac.jp
AF: Solar-Terrestrial Environmental Lab., Nagoya University, Honohara, Toyokawa, 442-8507 Japan
AB: When did the Earth's geomagnetic field first appear? Paleomagnetic studies trace the geomagnetic field back only to about 3.5 Ga ago [1]. It has been taken for granted that the development of the geomagnetic field was concomitant with the formation of the Earth's core, but the time of core formation is not well known, nor is the role of the inner core in the generation of the geodynamo, so this fundamental issue remains enigmatic. We show that the isotopic compositions of N implanted in ancient lunar soils may reflect admixture of solar wind and terrestrial atmospheric component, suggesting transport of atmospheric components to the Moon. The geomagnetic field would prevent any effective ion loss from the atmosphere [2], but if the geomagnetic field was absent in the early Earth the escape of ions from the ionosphere would be enhanced considerably, and a substantial amount of terrestrial atmospheric ions may have been transported to lunar soils. Therefore, we propose that close examination of N isotopic compositions in ancient lunar soils may provide clues concerning the time of the appearance of the geomagnetic field. The PVO (Pioneer Venus Orbiter) observations have suggested the loss of 1E6 - 1E8 O+ cm-2s-1 from the Venus atmosphere [3]. This substantial ion loss is understood to reflect direct interaction between the solar wind and the upper atmosphere in the absence of a permanent dipole field. For N+/O+ of about 0.02 around the ionopause [4], we would expect N+ loss of about 2E5 cm-2s-1. Venus's atmosphere may serve as a proxy for the primitive atmosphere in the early Earth, which was likely to consist predominantly of CO2/CO [5]. We thus expect that a similar amount of N+ would escape from the Earth in the absence of the geomagnetic field. Also considering that the Earth was much closer to the Moon in the first several hundred Ma [6], we infer from simple geometrical considerations that a few percent of escaping ions from the Earth's ionosphere may have directly hit the lunar surface. Isotopic inventory considerations suggest that the implantation of about 1E4cm-2s-1 of N of terrestrial origin would suffice to explain the hypothesized non-solar N in ancient lunar soils. This amount of non-solar N could be attributed to terrestrial N that was transported to the Moon if the Earth did not have a geomagnetic field. It is known that the dynamical coupling between the Earth and the Moon is so stable that the near side of the Moon has remained facing the Earth for nearly the whole history of the Earth-Moon system [7]. Therefore, we predict that a considerable amount of terrestrial N would be observable in near side lunar soils whose exposure times predate the commencement of the geomagnetic field, whereas soils from the far side should not show terrestrial components. We propose that comparison of N isotopic components in ancient lunar soils from the far side with those from the near side could help resolve the time of the appearance of the geomagnetic field. [1] Hale C.J. Nature, 399, 249, 1987. [2] Seki et al., Science, 291, 1847, 2001. [3] Kasprzak et al. JGR, 11, 175, 1991. [4] Grebowsky et al. JGR, 9055, 1993. [5] Abe Y., Lithos, 30, 223, 1993. [6] Abe M. et al., Proc. 30th Int. Geological Conf., 26, 1, 1997. [7] Murray C.D. and Dermott, S.F., Solar System Dynamics, Cambridge Univ. Press,1999.
DE: 0343 Planetary atmospheres (5405, 5407, 5409, 5704, 5705, 5707)
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 1521 Paleointensity
DE: 2134 Interplanetary magnetic fields
DE: 2152 Pickup ions
SC: U
MN: 2003 Fall Meeting